Amelogenesis
Imperfecta

What is
Amelogenesis Imperfecta?

Amelogenesis imperfecta (AI) (amelogenesis –
enamel formation; imperfecta – imperfecta) is
a relatively rare group of inherited disorders characterized
by abnormal enamel formation. The term amelogenesis
imperfecta is reserved for hereditary defects of enamel
that are not associated with defects in other parts
of the body or other health problems. The prevalence
of these conditions has been studied in only a few populations
and has been reported to range from 1 in 700 to 1 in
15,000 [1-4].

The AI enamel defects are highly variable and include
abnormalities that are classified as hypoplastic (defect
in amount of enamel), hypomaturation (defect in final
growth and maturation of enamel crystallites), and hypocalcified
(defect in initial crystallite formation followed by
defective growth) [5].
The enamel in both the hypomaturation and hypocalcified
AI types is not mineralized to the level of normal enamel
and can be described as hypomineralized. AI can be inherited
as an x-linked, autosomal recessive (AR), or autosomal
dominant (AD) condition.

The diagnosis and classification of AI has traditionally
been based on the clinical presentation or phenotype
and the inheritance pattern [6].
Although multiple gene defects responsible for causing
AI have been identified since 1990, most of the AI types
do not have a known molecular basis. As the molecular
defects causing AI are identified, the need for a molecular
based nomenclature increases. The rapid identification
of multiple mutations in multiple genes has lead to
the acceptance of standardized nomenclatures for reporting
AI associated mutations at the genomic, cDNA and protein
level [7]. The most widely
accepted classification system for delineating the AI
types subdivides AI into four main types based on the
enamel defects and then further divides them into14
distinct subtypes based on clinical appearance (phenotype)
and mode of inheritance [6].

Types
of Amelogenesis Imperfecta

TYPE I: HYPOPLASTIC
AI

All of the hypoplastic AI subtypes are characterized
by the primary feature of deficient amount of enamel
formed. The decreased amount of enamel varies in the
different subtypes and can be characterized by enamel
that is pitted, has grooves or furrows, has large areas
of missing or very thin enamel surrounded by more normal
enamel, or enamel that is very thin over the entire
tooth crown. Witkop’s clinical classification
of hypoplastic AI recognizes seven subtypes [6].

The four main AI types are shown in Table 1 and brief
descriptions of the 14 subtypes are provided. After
the name of each subtype are the letters designating
mode of inheritance (e.g. AD = autosomal dominant, AR
= autosomal recessive, and X-linked). For additional
explanation of these different modes of Mendelian Inheritance
see the section on Mendelian Inheritance.

Clinical
and Hereditary Characteristics of Four Main AI
Types (Table
1) [8]

Type

Clinical
Appearance

Enamel
Thickness

Radiographic
Appearance

Inheritance

Hypoplastic
(Type I)

Crowns size
varies from small to normal, small teeth may lack
proxmial contacts, color varies from normal to
opaque white – yellow brown

Varies from
thin and smooth to normal thickness with grooves,
furrows and/or pits

Enamel has
normal to slightly reduced contrast/ thin

Autosomal dominant,
recessive, or X-linked

Hypomaturation
(Type II)

Varies from
creamy opaque to marked yellow/brown, surface
of teeth soft and rough, dental sensitivity and
open bite common

Normal thickness
with enamel that often chips and abrades easily

Enamel has
contrast similar to or > than dentin, unerupted
crowns have normal morphology

Modes
of Mendelian Inheritance Associated with AI

On average, half of the offspring of an affected
individual will be affected. There is a 50% chance
for the child of an affected individual to be affected.

Affected males and females have similar clinical
presentation.

Autosomal
Recessive Inheritance

Unaffected parents will have affected offspring.

On average, one in four offspring of carrier parents
will be affected.

More likely to occur when parents are related (consanguineous
relationship).

X-linked
recessive inheritance

Do not have male to male transmission.

All daughters of an affected male are carriers.

Half of the sons born to a carrier female will be
affected.

Affected males have more severe manifestations than
females.

Females can show no manifestations to severe manifestations
due to lyonization. Females express only one X chromosome
per cell with the other X chromosome becoming the
bar body. If adequate numbers of cells express the
X chromosome carrying the mutant allele, they will
have varying degrees of the enamel defect.

Amelogenesis
Imperfecta Research

Since the first enamel matrix
associated gene (AMELX) was identified in the
late 1980’s [26]
and the first AI associated mutation was reported in 1990
[36, 37], there have been
over a 25 different AI associated mutations identified.
This research confirms the genetic heterogeneity of AI
and has greatly advanced our understanding of these conditions.
There are now known to be mutations in four different
genes that are associated with AI (i.e. AMELX, ENAM,
KLK4, MMP20). Although a DLX3 mutation has
been reported as causative of hypomaturation AI with taurodontism
this mutation is actually asscociated with a variant of
the tricho-dento-osseous syndrome. Linkage studies
on large families with autosomal dominant AI show no evidence
of linkage to any of these or other known AI candidate
genes indicating that other, as yet to be discovered,
genes will be associated with AI.

As the genetic mutations associated with the different
AI types become known, our ability to accurately diagnose
the different AI conditions is becoming markedly improved.
The similarity in clinical features makes differentiating
some AI types at the clinical level difficult. Correlation
of the genetic mutations with the clinical phenotypes
of each AI subtype will be invaluable for managing patients
with AI. This knowledge also allows us to better predict
which AI types are associated with problems such as
calculus formation and skeletal open bites and for selecting
the most optimal treatment approaches.

Amelogenin (AMELX)
Mutations

Over a 14 mutations have been identified in the AMELX
gene. Different phenotypes are associated with different
AMELX mutations depending on the mutations
effect on the amelogenin protein. All the AMELX
mutations resulting in a loss of the C-terminus of the
protein or total loss of the protein (signal peptide
mutations and large deletions) have hypoplastic enamel.
Mutations causing single amino acid changes cause hypomaturation
defects that in some cases also have hypoplastic enamel.
For a review of the phenotype-genotype associations
in X-linked AI see Wright et al. 2003 [38].

The AMELX mutations are presented in Table
2 using a standardized nomenclature [7].
The genomic, cDNA and deduced protein are shown in the
first three columns. The accession numbers for the genomic
and cDNA sequences are shown below the table.

Mutations in the ENAM gene are now known to
be associated with at least two clinically distinct
AI types. These traits are most often transmitted in
an autosomal dominant manner with variable expression
being noted. One recent family was reported to be transmitted
as autosomal recessive. Four different mutations have
been identified to date. All of these mutations result
in hypoplastic AI. Mutations resulting in loss of most
of the protein such as the p.K53X result in local pitting
presumably due to haploinsufficiency. In contrast mutations
that cause a substantion portion of an altered protein
to be translated produced a generalized thin enamel
phenotype presumably due to a dominant negative effect.
The ENAM mutations are shown using a standardized
nomenclature in Table 3.

ENAM
Mutations (Table 3)

Genomic
DNA^

cDNA+

Protein*

Phenotype

Reference

g.2382A>T

c.157A>T

p.K53X

Local hypoplastic

Mardh [11]

g.6395G>A

IVS7+1G>A; c.534+1G>A

p.A158_Q178del

Generalized thin hypoplastic

Rajpar [13]

g.8344delG

IVS8+1delG;c.588+1delG

p.N197fsX277

Generalzied thin hypoplastic

Kida 2003 [14] Hart 2003 [15]

g.13185_13186insAG

c/1258_1259insAG

p.P422fsX448

Generalized thin hypoplastic

Hart 2004 [16]

a Reference sequence GenBank accession
no. AY167999; the A of the initiator ATG is taken as
+1.
b Reference sequence GenBank accession no. AF125373;
the A of the initiator ATG is taken as +1.
c Initiator methionine as the +1 position.

Enamelysin (MMP20)
and Kallikrein 4 (KLK4)Mutations

Mutations in the two major proteinases (MMP20
and KLK4) involved in processing the enamel
matrix during development and mineralization are associated
with autosomal recessive pigmented hypomaturation amelogenesis
imperfecta. The enamel is of normal thickness but has
a reduced mineral content and increased protein content.
The trait is transmitted in an autosomal recessive manner.
MMP20 codes for a matrix metalloproteinase
that is highly expressed during the secretory stage
of enamel formation. Kallikren 4 codes for a serine
proteinase that is maximally expressed by ameloblasts
during the maturation stage of enamel development. While
mutations in both the major proteinases have been identified,
evaluation of families with autosomal recessive pigmented
hypomaturation AI have not shown mutations in these
two genes suggesting this AI subtype is genetically
heterogeneous.

MMP20
and KLK4 Mutations (Table
4)

MMP20
Mutations

Genomic
DNA^

cDNA+

Protein*

Phenotype

Reference

g.30561A>T

c.954-2A>T or c.IVS6-2A-T

p.I319Fs338X or p.I319X

Pigmented Hypomaturation Decreased Mineral

Kim et al. 2005

g.16250T>A

c.678T>A

p.H226Q

Hypomaturation

Hart et al. in press

KLK4
Mutations

g.2142G>A

c.458G>A

p.W153X

Pigmented Hypomaturation Decreased Mineral

Hart et al. 2004

a Reference sequence GenBank accession
no AF228497; the A of the initiator ATG is taken as
+1.
b Reference sequence GenBank accession no NM_004917;
the A of the initiator ATG is taken as +1.
c Initiator methionine at the +1 position.

FAM83H

The most recently identified
gene shown to cause AI is the FAM83H gene. This
gene is located on chromosome 8q24 and causes autosomal
dominant hypocalcified AI (ADHCAI) in families around
the world [63-65].
The importance of the FAM83H gene is evident from
the resulting phenotype that is characterized by
a marked reduction in mineralization of the enamel
giving rise to the designation as hypocalcified.
The discovery that FAM83H causes ADHCAI provides
new knowledge of the molecular basis of the most
common AI type in North America. In our population
ADHCAI accounts for 25% the families with known
AI types and 35% of the individuals with AI.

The role of the FAM83H gene and protein during
enamel formation remains unknown. The gene is
expressed in many tissues, however, all mutations
reported to date result only in enamel abnormalities,
suggesting this gene is essential for enamel formation
but not as critical in other tissues [1,
4]. FAM83H codes
for an 1179 amino acid protein whose function
is unknown. A potential transactivation domain
was identified in silico, suggesting that FAM83H
could serve as a transcription factor.

Characterization of the phenotype in ADHCAI is
variable within and between families and we have
identified different ADHCAI phenotypes associated
with specific FAM83H mutations. Most individuals
with FAM83H mutations are characterized by both
the primary and permanent teeth having a yellow
brown discoloration and enamel that tends to readily
fracture from the teeth (generalized phenotype).
The enamel has a variable but markedly decreased
mineral content (40 to 70% mineral per volume)
and an increased protein content that is not proline
rich as is seen in the hypomaturation AI types
[5]. Individuals
having mutations causing truncation of the protein
at or prior to amino acid 677 all display a generalized
ADHCAI. Interestingly, individuals with mutations
affecting the FAM83H protein at amino acid 694
or later have a localized ADHCAI that affects
primarily, but not exclusively the cervical third
of the crown [3].
We hypothesize that this unique phenotype is associated
only with mutations that result in production
of a longer and potentially less dis-functional
FAM83H protein compared with mutations truncating
the protein at amino acid 677 or lower that result
in generalized ADHCAI. We believe the generalized
ADHCAI results from a dominant negative effect
and have shown that the mutant mRNA is not degenerated
by non-sense medicated decay. There are now 13
mutations reported in FAM83H that cause ADHCAI
as shown in Table 5.

FAM83H
Table 5

cDNA

Protein

Phenotype

Reference

c.860C>A

p.S287X

Generalized

Wright 2009

c.891T.A

p.Y297X

Generalized

Lee 2008

c.923_924delTC

p.L308fsX323

Generalized

Wright 2009

c.973C.T

p.R325X

Generalized

Kim 2008

c.1192C.T

p.Q398X

Generalized

Kim 2008

c.1243G.T

p.E415X

Generalized

Lee 2008

c.1330C.T

p.Q444X

Generalized

Hart 2009

c.1366C.T

p.Q456X

Generalized

Hart 2009

c.1380G.A

p.W460X

Generalized

Lee 2008

c.1408C>T

p.Q470X

Generalized

Wright 2009

c.1872_1873delCC

p.L625fsX703

Localized

Wright 2009

c.2029C.T

p.Q677X

Generalized

Lee 2008

c.2080G>T

p.E694X

Localized

Wright 2009

The Figure below shows the difference
between localized and generalized HCAI phenotypes
and the different protein designations associated
with each phenotype.

Figure 5

Table of AI Types and Currently Known Genes

Witkop Classification

Modified Classification

OMIM #

Gene

TYPE I Hypoplastic

IA-hypoplastic pitted (AD)

AD generalized hypoplastic

?

LAMB3

IB-hypoplastic local (AD)

AD generalized hypoplastic

104500

ENAM

IC–hypoplastic local (AR)

AR local hypoplastic

204650

ENAM

ID-hypoplastic smooth (AD)

AD generalized thin hypoplastic

?

?

IE-hypoplastic smooth (X linked)

X linked generalized thin hypoplastic

301200

AMELX

IG—enamel agenesis (AR)

AR generalized thin hypoplastic (enamel-renal syndrome)

614253 204690

FAM20A

TYPE II Hypomaturation

IIA—pigmented hypomaturation (AR)

AR pigmented hypomaturation

(IIA1) 204700 (IIA2) 612529 (IIA3) 613211 (IIA4) 614832

MMP20 KLK4 WDR72 C4ORF

IIB-hypomaturation (X linked)

X linked hypomaturation

301200

AMELX

IIC-snowcapped (X linked)

X linked snowcapped

301200

AMELX

TYPE III Hypocalcified

IIIA- hypocalcified (AD)

AD hypocalcified

130900

FAM83H

IIIB—hypocalcified (AR)

AR hypocalcified

Not listed

SLC24A4

TYPE IV Hypomaturation-Hypoplastic with Taurodontism

IVA Hypomaturation-Hypoplastic with Taurodontism (AD)

(AD) Tricho-dento-osseous syndrome

AI 104530 TDO 190320

DLX3

IVB- Hypomaturation- Hypoplastic with Taurodontism (AR)

?

?

?

For additional information on the AI associated genes
and molecular defects in AI see the OMIM
Web site.